When me and my sister packed her kids up in my truck just this last few weekends ago, we were unable to seat her car seat in the right rear most passenger seat in my 2004 extended cab silverado, for the seat belt would not deploy, it was stuck in its upright, returned position. No coaxing or yanking would release it.

is there a button or some greasing that needs lube somewhere that will release the belt? In my state all passengers are required to wear belts and this has me semi-worried. (there have never been enough passengers in the rear to warrant the misuse of this belt, its hardly ever been used)

Which seat belt is this? The right-rear passenger side seat. Can you see if there was anything that got caught up in there that's causing extra tension? It's possible that a piece of string or a piece of food (I've seen that happen myself actually) got caught up inside of there and made it very hard to get apart.

To get more slack you're going to need a torx-wrench to get the thing off. It's possible also that the retractor lock is stuck when the momentum shifts forward. Maybe try to pound on it a little bit??? just a thought.

I have seen many get stuck because of a twist in the belt itself. If you use a large torques bit and remove the top "d" ring that may let it retract enough that you can get it moving again. You want to make sure that there are no twists in the belt and reinstall the "d" ring on the rear pillar. After you have one twist in the belt it will occur more often. My brother designs and makes the seat belts and mechanisms for most GM vehicles and when I had the same issue he gave me the above instructions to fix it. Mine kept retwisting and getting stuck, so he gave me a new belt and mechanism. From what he told me, Most belt manufactures would rather replace the belt for you than have you go to a dealer. If you take it to a dealer it goes on some sort of federal safety report that their belt failed. I can ask him if there is a release button of some sort.

I think that the mechanism that locks the belt incase of force is locking the belt. Because a small amount of slack is allowed then it locks, exactly like how the locking mechanism acts when its engaged.

I will try the above about taking a torx bit and removing the top of the belt and leting some slack recess and then trying to pull it back out, in fact that will likely either fix the problem, or help isolate it as two things will happen, it will unlock or it will shorten the amount of slack left.